Glitch delays space station crew's return to Earth (Update)

The Soyuz capsule failed Friday to undock for the first time in a decade of flights to the International Space Station, forcing three crew members to remain an extra day in orbit.

The Russian mission control centre near Moscow said the shuttle's return to Earth was rolled back to Saturday over fears that the capsule was not fully airlocked after a computer malfunction.

"The landing of the Soyuz TMA-18 spacecraft and crew... has been pushed back by 24 hours to Saturday September 25 due to technical problems," Russia's space agency chief Anatoly Perminov said in a statement.

The new plan calls for the Soyuz crew to undock at 5:59 am Moscow time (0159 GMT) and land in the central Kazakh steppes at 9:23 am (0523 GMT), a mission control official told the Interfax news agency.

US astronaut Tracy Caldwell Dyson and Russian cosmonauts Alexander Skvortsov and Mikhail Kornienko had been due to make their fiery descent locked in the capsule on Friday morning, after more than six months in orbit.

But the manoeuvre was first delayed by a few hours due to "small glitches", before being cancelled outright on Friday, Russian space officials said.

When the crew attempted to undock from the International Space Station (ISS) one of their computers sent up a red flag -- showing the airlock was not fully sealed, Roskosmos chief Perminov explained.

"The onboard computer system is picking up a false signal that there is no airlock on the station after the hatch is closed," Perminov said.

"We have carried out checks on the air tightness. The airlock is confirmed on the ISS and the Soyuz, which is the most important thing for today."

Perminov stressed that the Soyuz crew members were in no immediate danger and had all rejoined the three other crew members onboard the ISS to prepare for the new landing schedule.

Russian cosmonaut Fyodor Yurchikhin and two NASA astronauts, Doug Wheelock and Shannon Walker, are due to remain on the space station.

"We could have done it (the undocking) today but we need extra time to avoid further risks. There is no reason to rush. The most important thing is to guarantee the safety of the crew," he added.

"We need to figure out completely the reason for the false signal and fully guarantee that the dynamic processes of the operation are safe."

In the event of another computer bug, the crew will pilot the undocking manually, according to a specialist with Roskosmos' human space flight programme.

"If tomorrow the automatic system again does not allow the Soyuz to undock, cosmonauts will shift to a manual work regime," the unnamed specialist was quoted by Interfax as saying.

Friday's incident was the third docking problem at the station in four months after the automatic system failed on two unmanned Russian Progress supply shuttles, causing one to fly past the station in June.

The string of mishaps in a space programme that usually strives for and achieves pinpoint accuracy comes just before NASA mothballs its shuttle later this year, leaving the ISS entirely dependent on the Russian Soyuz.

"It's a regrettable situation which should not have occurred with a system that has always functioned well," Igor Lisov, an expert with leading Russian space science journal Novosti Kosmonavtiki, told AFP.

"It's the first time that the Soyuz has been unable to undock from the ISS."

But a space industry expert cited by the ITAR-TASS news agency revealed that the Soyuz had already had troubles undocking in May, although he said these were swiftly resolved.

He added that the problems may be linked to a recent shift to a "digital" flight piloting system.

"Its special feature is that if the cosmonauts are convinced that a gauge is giving a false reading, they can't shift to manual operations. Only the mission control can give that command to the piloting system," he said.